There are a number of applications for which the temperature of the fluid being used in a particular process must be maintained within very close tolerance of a predetermined value. For example, in photographic film developing, variations in the temperature of the water used in the developing process adversely affect the quality of the finished product. The same is true for a large number of other industrial processes. In still other applications, controllable variation of the fluid temperature is required. For example, in some rehabilitative procedures for persons with bone fractures, the injured limb is alternately bathed with hot and cold water. For such an application, a closely regulated pattern of fluid temperature as a function of time is desirable. A common requirement for all of these applications is automatic control of the fluid temperature such that the desired temperature is maintained regardless of fluctuations in the hot and cold fluids which are supplied to the system.
A number of arrangements have been proposed in the past for mixing hot and cold fluids together to produce a mixed fluid having a preselected temperature. Many of those proposed systems employ analog circuitry to provide the feedback control algorithm. For example, U.S. Pat. No. 4,359,186 issued to Kiendel discloses a mixing valve arrangement which utilizes motor driven valves to supply hot and cold water to a mixing chamber. The temperature of the water in the mixing chamber is measured and is used, along with the flow rate of fluid moving through the mixing chamber, in an analog control circuit which provides signals for controlling the valve motors in response to temperature variations. The type of temperature control system disclosed by Kiendel suffers from the inflexibility which is typical of analog control circuits. In order to adjust the control constants or to change the control algorithm, circuit components must be physically replaced or adjusted.
Other practitioners have apparently recognized the usefulness of a digital processor such as a microcomputer in implementing the control algorithm. For example, U.S. Pat. No. 4,420,811 issued to Tarnay et al discloses a water temperature control system for which the feedback control algorithm is preferably embodied in a microcomputer. However, the patent to Tarnay et al is apparently not particularly concerned with rapid and accurate system response. The valve arrangement, the configuration of the water discharge channel, and the temperature sensor device taught by Tarnay et al all appear to create inaccuracies and slowness in the system response time which would be difficult to solve using the arrangement disclosed in their patent.
U.S. Pat. application Ser. No. 837,500, filed Mar. 7, 1986 by the present inventor, and now issued as U.S. Pat. No. 4,931,938, discloses a microcomputer controlled faucet which rapidly and accurately controls hot and cold supply valves so as to maintain the temperature of the water being discharged from the faucet within a very small margin of error of the preselected temperature. The system disclosed utilizes a mixing connection which is configured to promote even mixing of the hot and cold fluids, and also a fast response temperature sensor placed in a location that provides accurate sensing of the mixed fluid temperature. As shown by the sample operational results included in that patent application, the microcomputer controlled system disclosed therein provides excellent control of the fluid mixture temperature, even when the hot and cold fluid supplies vary significantly in temperature and pressure.
The present inventor has unexpectedly discovered that, for some applications, the fluid temperature control system is further improved by actively mixing the hot and cold fluids together before measuring the temperature of the fluid mixture. By appropriately doing so, the control system of the present invention provides a control temperature tolerance of plus or minus 0.2 degrees Fahrenheit.
In accordance with the foregoing, it is an object of the present invention to provide a fluid system which automatically maintains the temperature of the fluid mixture discharged from the system at a preselected value, within predetermined limits.
It is a further object of the present invention to provide a temperature control system which accurately and rapidly adjusts the flow rates of hot and cold fluids into the system so as to maintain the temperature of the mixed fluids within a very close tolerance of the desired temperature.
It is yet another object of the present invention to provide a fluid control system for which the feedback algorithm can easily be changed.
It is also an object of this invention to provide a temperature controlled fluid system which is reliable in operation, rugged in construction, and economical to manufacture.